What does MAILER DAEMON mean in my email?

I think i have some kind of virus in my email, i keep getting emails from something called MAILER DAEMON@YAHOO and also from POSTMASTER@HOTMAIL. its driving me mad! sometimes i will get 40 or 50 emails in the space of 2 Minuit's all saying something about failed messages and not being able to deliver messages with addresses i have ever seen before and emails i have not sent, i get hundreds every day, this morning when i checked i had over 1500 of these emails, they even go to my spam box. What is it? what does it mean and how do i get rid of it? also, if it is a virus, why isn't my anti virus stopping it?